magical formula, 1560s, from Latin (Q. Serenus Sammonicus, 2c.). According to OED, "The etymology of post-classical Latin abracadabra has been the subject of much conjecture; no documentation has been found to support any of the various conjectures which have been put forward."

A common theory that dates at least to 17c. connects it with the Gnostic deity Abraxas; another possibility is a mistranscription of an unfamiliar magical formula into Latin letters (as in athame.) Compare ananizapta, a magical word attested from mid-15c. Another variant taken from ancient Greek is akrakanarba (1922).